1. Field of the Invention
The present invention generally relates to optical imaging systems, and more specifically to a color switching apparatus for sequentially illuminating a liquid crystal light valve in an optical projector with light beams of three primary colors.
2. Description of the Related Art
An article entitled "Reinventing the color wheel", by T. Haven, appearing in Information Display, January 1991, pp. 11-15, describes the recent history of "field-sequential-color" optical imaging systems. The basic principle is to sequentially produce three monochrome images corresponding to three primary color components or fields of a scene, and switch three primary color filters into the image path in synchronism with the three monochrome images respectively. If this is performed at a fast enough rate, the color components will be averaged and appear to merge over time to synthesize a color image of the scene without apparent flicker.
In the early systems, a color wheel including red, green and blue filters was rotated by an electric motor in front of a monochrome television screen in synchronism with electronic switching of the monochrome color component fields applied to the screen. Although operable, this approach suffered from the drawbacks of excessive mechanical system size, technical deficiencies including difficulty of synchronization, and incompatibility with monochrome television system standards.
The improved arrangement described in the referenced article replaces the rotary color wheel with a color filter switching device including color and monochrome polarizers and liquid crystal or "pi" (.pi.) cells which are electrically switchable to provide either 90.degree. (half-wavelength) polarization rotation, or no polarization rotation. The monochrome image fields are applied to a cathode ray tube (CRT) having a white phosphor.
The color polarizers separate the white phosphor from the CRT into red, green and blue color components having different polarizations. The pi cells are switched such that the selected color component is allowed to pass through the device, but the unwanted color components are rotated to a polarization state which is blocked by the monochrome polarizer.
A major drawback of this arrangement is that only about 6.5% of the light from the CRT is transmitted through the switching device. This is extremely inefficient, and requires a CRT with inordinately high luminance to produce an acceptably bright image. The sheet polarizers are also highly absorptive, and cannot be used with a high intensity light beam.
The liquid crystal light valve (LCLV) is an optical-to-optical image transducer that is capable of accepting a low-intensity visible light image and converting it, in real time, into an output image with light from another source. A general description of the applications of LCLV technology is presented in a paper entitled "Progress in Liquid Crystal Light Valves", by W. P. Bleha, Laser Focus/Electro-Optics, October 1983, pp 111-120.
The LCLV is widely used as an element in optical projectors. In such a system, the LCLV is optically modulated with an image from a CRT to provide a reflectance pattern on a surface thereof corresponding to the image. The surface is illuminated with white light from a high intensity source, and an image of the illuminated surface pattern is projected onto a screen by an optical system.
LCLV projectors are capable of providing large, high intensity monochrome images with excellent resolution. However, to make a color projector using prior art technology, it would be necessary to provide three monochrome projectors with color filters and an arrangement for accurately registering the color images from the monochrome projectors. This would be too large and expensive for practical use.